PET (Positron Emission Tomography) is a method for injecting radiopharmaceuticals labeled with a positron emission nuclide into the body to image the spatial and temporal distribution of the radiopharmaceuticals. In particular, PET scanning in which a radiopharmaceutical called fludeoxyglucose (FDG) is used has become the focus of attention because of the usefulness in making an early diagnosis of cancers over the entire body.
In PET, a radiopharmaceutical to be injected is selected to obtain information on various functions of the brain and organs. It was, however, difficult to accurately localize a site of cancer, if the cancer was found, due to the shortage of anatomical information. It is described that X-ray CT images are structural images, while PET images are functional images. In response to the above-described demands, PET/CT scanners capable of performing PET and X-ray CT scanning continuously on the same bed have been made commercially available from many companies, thus greatly contributing to the widespread use of FDG-PET.
On the other hand, it is also important to treat cancers found by a PET diagnosis or others. A method for treating cancers by using nuclear radiation, unlike conventional surgical procedures or chemotherapies has become the focus of attention. In particular, particle radiotherapy in which heavy ion particle beams or proton beams are irradiated only at a cancer site has gained a great deal of attention as a method for providing excellent therapeutic effects and characteristics of acutely concentrated irradiation to lesions. Irradiation is performed by controlling accurately the direction and dosage of beams according to treatment plans carefully calculated on the basis of X-ray CT images which have been taken separately. However, in reality, it is difficult to confirm accurately whether irradiation has been performed in accordance with treatment plans or not. If the patient is positioned wrongly to result in deviation of the irradiation field, the deviation is not easily detected. Therefore, a method in which PET is used to monitor the irradiation field of particle beams in real time is now gaining attention. According to this method, a PET radiopharmaceutical is not injected but annihilation radiation resulting from projectile fragmentation reactions or target nuclear spallation reactions by beam irradiation is imaged by using the principle of PET. Since a site at which the annihilation radiation is generated is strongly correlated with the dosage distribution of irradiation beams, this method is expected to monitor treatment.
The principle of PET is as follows. As shown in FIG. 1, positrons emitted from a positron emission nuclide 8 by the decay of positrons undergo pair annihilation with electrons in the vicinity, and the thus generated pair of annihilation (gamma) photons 8a, 8b at 511 keV are determined by a pair of radiation detectors 10a, 10b according to the principle of coincidence. Thereby, the position at which the nuclide 8 is present can be localized on one line segment connecting between the pair of detectors 10a, 10b (coincidence line: line-of-response: LOR). When an axis from the head of a patient to the feet is defined as a body axis, a distribution of the nuclide on a planar surface intersecting perpendicularly with the body axis is obtained by image reconstruction in two-dimensional mode from data of the coincidence line determined on the planar surface in various directions.
Therefore, as shown in FIG. 2 (A) covering a polygonal-type PET scanner and in FIG. 2 (B) covering a ring-type PET scanner, earlier PET scanners were provided with a constitution to arrange detectors 10 on a planar surface which was given as a field-of-view (FOV) in such a manner as to surround the FOV in a polygonal shape (A) or a ring shape (B). In FIG. 2 (B), numeral 6 depicts a patient and that of 11 depicts a detector ring.
In the 1990s, as illustrated in FIG. 3 (A) covering a multi-layer polygonal-type PET scanner and in FIG. 3 (B) covering a multi-layer ring-type PET scanner, 3-D mode PET scanners were developed one after another in which detector rings 11 were arranged in the body axis direction of the patient 6 to give a multiple ring 12, thereby a FOV in two-dimensional mode was changed to that in three-dimensional mode, and the coincidence was also determined between the detector rings 11 to increase the sensitivity greatly.
On the other hand, as illustrated in FIG. 4, gamma camera opposition-type PET scanners which rotate gamma cameras 14 arranged in opposition have also been developed. However, this type of PET scanner is insufficient in sensitivity due to the limited solid angle of a detector, with no widespread use. Positron imaging equipment in which cameras are not rotated has been commercially available mainly for experimental uses. The equipment is to obtain planar images parallel with the face of the detector and not for tomography (corresponding not to X-ray CT but to radiography in X-ray equipment).
In order to increase the resolution of an image in view of the principle of image reconstruction, it is necessary that coincidence lines are obtained densely.
The detector sensitivity of a PET scanner is important in increasing the accuracy of an image. The detector sensitivity is generally considered insufficient. In order to compensate for the insufficiency, the dosage of a injecting nuclide at about 5 mCi (=185 MBq) (an effective dosage equivalent to about 40 times higher than that used in X-ray photograph of the chest) and the scanning time which is long, about 30 minutes are required. These factors cause mental and physical burdens to patients and are also one of the reasons that medical institutions cannot lower examination costs.
Therefore, in order to increase the detection sensitivity, recently developed PET scanners tend to array detectors, with no clearance left therebetween, and also arrange them long in the body axis direction. A patient port 13 (refer to FIG. 3) is approximately about 60 cm in diameter and from 40 cm up to 100 cm long in the body axis direction.
However, there is such a problem that a long patient port further increases the closed nature of the port, thereby causing psychological stress to patients. In particular, a PET scanner is frequently used not only in cancer screening tests for healthy people but also in examinations for patients having various types of diseases including mental disorders. Therefore, it is strongly desired to reduce the psychological stress to patients. A situation in which a patient under PET scanning is not visually confirmed for health conditions is not desirable also for medical personnel who perform PET scanning. Further, in research for understanding brain functions, many experiments are conducted in which blood samples are taken at intervals of several minutes during PET scanning or visual stimulations are given to visualize reactions inside the brain by using PET. The long patient port causes problems in these experiments as well.
Further, where a PET scanner is used to monitor particle radiotherapy in real time, not only a site to be treated can be determined during the same session by the PET scanner but also a PET scanner is required to be high in sensitivity because annihilation radiation resulting from irradiation is at a trace amount as compared with the amount of nuclide injected on ordinary PET scanning. In order to realize the high sensitivity, detectors must be arranged densely and extensively. However, since the detectors are not to block particle beams, it was difficult to arrange the detectors on the PET scanner in such a manner as to meet simultaneously the above-described two conditions. In the Gesellschaft für Schwerionenforschung mbH (GSI) of Germany and the National Cancer Center (Hospital East) of Japan, the rotational opposition-type PET scanner shown in FIG. 5 (A) is used to monitor treatment. Gamma camera opposition-type PET scanners can be easily arranged so as not to block particle beams 22 irradiated from a therapeutic radiation controller 20 but definitely hold a disadvantage in that the detector sensitivity is fundamentally lower.
A research group in Germany has proposed a fixed slit-type PET scanner as shown in FIG. 5(B) in which a slit 12s is made on the side of a multilayer ring PET scanner for allowing beams to pass through, making evaluation based on the computer simulation. However, the slit lacks necessary information for image reconstruction because of the presence of the slit, resulting in the deteriorated quality of an image, which is regarded as a problem (P. Crespo, et al., “On the detector arrangement for in-beam PET for hadron therapy monitoring,” Phys. Med. Biol. Journal, vol. 51 (2006) pp. 2143-2163).
FIG. 6 illustrates a representative constitution of a conventional PET/CT scanner (refer to U.S. Pat. No. 6,490,476 B1). In this drawing, the numeral 32 depicts an X-ray tube of the X-ray CT scanner 30, and the numeral 34 depicts an X-ray detector, both of which are rotated to perform scanning. There is now available a type in which each of the PET scanner and the X-ray CT scanner is provided with a completely independent gantry and a type in which they are housed into one gantry in an integrated manner. The PET scanner and the X-ray CT scanner may be arranged in a different order, depending on the type, but they are always arranged in tandem inside the gantry. In terms of the movement of a bed 7, there is a type in which a gantry moves with respect to a fixed bed and a type in which a bed moves with respect to a fixed gantry.
Whichever the type may be, in a conventional PET/CT scanner, a field of view (FOV) of the PET is not in agreement with that of X-ray CT, or is several dozen centimeters apart from each other. There is found a potential problem that the same site is not determined during the same session by PET and X-ray CT. In the conventional PET/CT scanner, a bed is moved relatively with respect to a gantry, that is, a temporal difference is given, by which the same site can be imaged by PET and X-ray CT. In currently available FDG-PET check-ups, scanning is performed in several minutes at each site due to the fact that a radiopharmaceutical moves slowly in the body and the sensitivity is lower. For this reason, the above-described temporal difference is not recognized as a problem. However, a discrepancy between PET images and X-ray CT images on the chest which entails the deformation by respiration is found. This discrepancy is now recognized as a serious problem.
Thanks to the recently advanced development of PET scanners and PET radiopharmaceuticals, new PET radiopharmaceuticals and PET scanners extremely high in sensitivity will become available. Therefore, increased demand can be expected for imaging the pharmacokinetics inside the body at a higher speed. In this instance, the above-described temporal difference may be recognized as a problem.
A FOV of the conventional PET scanner in the body axis direction is limited to about 20 cm. Therefore, a bed is moved over several dozen minutes intermittently or continuously to image an entire body of a patient. Therefore, a site apart at a greater distance than the FOV in the body axis direction cannot be imaged theoretically during the same session. Although there are problems such as complicated data processing, the greatest reason for the limited FOV in the body axis direction is the increased equipment cost due to a greater number of detectors. On the other hand, there is a strong demand for expansion of the FOV in the body axis direction. For example, the Research Institute for Brain and Blood Vessels Akita conducted research in which two commercially available PET scanners were arranged together to image brain and heart regions during the same session and independently from each other (H. Iida, et al., “A New PET Camera for noninvasive quantitation of physiological functional parametric images. HEADTOME-V-Dual.,” Quantification of brain function using PET (eds. R. Myers, V. Cunningham, D. Bailey, T. Jones) p. 57-61, Academic Press, London, 1996).
In the above constitution, although the FOV in the body axis direction is expanded intermittently, an increasing number of detectors are installed to raise the cost. Further, since each of the PET scanners makes an independent coincidence determination, no detection can be made for the radiation from a nuclide present at a region between the scanners, thus resulting in a failure in imaging the region between the scanners.
In positron imaging equipment not for tomography but for planar imaging, an idea has been proposed that detectors are arranged at sparsely spaced clearances, thereby sampling of the coincidence line can be sparsely performed to increase the uniformity and also expand a FOV (Japanese Published Unexamined Patent Application No. Hei 9-211130 and Japanese Published Unexamined Patent Application No. 2001-141827).
However, where this idea is applied as it is to a PET scanner so that detectors are arranged sparsely on a ring, the coincidence line necessary for image reconstruction is lacking to inevitably result in the deteriorated quality of an image.
On the other hand, Japanese Published Unexamined Patent Application No. Hei 5-150046 has proposed a method in which a FOV different from that which is imaged by a PET scanner as a tomographic image is measured simply at low cost as a projected image. As illustrated in FIG. 7, this method assumes such a case that the head of a subject 101 is measured by using a PET scanner (detector 102) and the heart is measured during the same session for the conditions by using another device (detector 105). This method may have demand for an activation test or the like in which, for example, 15O-labeled water is injected to measure the change in local cerebral blood flow in response to stimulation. Images of the brain must be obtained as tomographic images. Regarding the heart, only monitoring of blood flow pumped by the heart will be sufficient. Therefore, this method is considered to assume that a PET scanner for obtaining tomographic images and positron imaging equipment for obtaining projected images are arranged in tandem. Specifically, the PET scanner needs the image reconstruction for obtaining tomographic images, while the positron imaging equipment does not need the image reconstruction because measured data in itself is a projected image. In this method, some of the detectors used in the PET scanner also act as detectors of the positron imaging equipment, thereby providing an advantage that these two sets of equipment are combined into one set of integrated equipment to reduce the cost. In FIG. 7, the numerals 103 and 106 depict coincidence circuits, and those of 104 and 107 depict data processors.
The above method is to expand a FOV, however, it may be considered to secure an open space from a different point of view. However, provided by the detector 5 is not a tomographic image but a projected image.
Japanese Published Unexamined Patent Application No. Hei 5-150046 has also proposed that a plurality of detectors 5 are arranged to pick up projected images at the same time in various directions but has not described a point where an image is reconstructed to obtain a tomographic image.